docs: def url(self):
          '''
          Because invoice URLs are generally emailed, this
          includes the default site URL and the protocol specified in
          settings.
          '''
          if self.id:
              return '%s://%s%s' % (
                  getConstant('email__linkProtocol'),
                  Site.objects.get_current().domain,
                  reverse('viewInvoice', args=[self.id,]),
              )